Pulp chamber temperature rise in light-cure bonding of brackets with and without primer, in intact versus restored teeth

ABSTRACT Objective: To evaluate the pulp chamber temperature rise (PCTR) in light-cure bonding of brackets with and without primer, in intact and restored mandibular central incisors (M1), maxillary first premolars (Mx4), and mandibular third molars (M8). Material and Methods: Ninety human teeth were included: M1 (n=30), Mx4 (n=30), and M8 (n=30). Light-cure bonding of brackets was performed in intact (n=60) and restored (n=30) teeth, with primer (n=60) or without (n=30) primer. PCTR was defined as the difference between initial (T0) and peak temperatures (T1), recorded with a thermocouple during light-cure bonding. Differences on PCTR between bonding techniques (primer vs. no primer), teeth types (M1 vs. Mx4 vs. M8), and teeth condition (intact vs. restored) were estimated by ANCOVA, with α=5%. Results: PCTR was significantly higher with the use of primer (2.05 ± 0.08oC) than without primer (1.65 ± 0.14oC) (p=0.02), and in M1 (2.23 ± 0.22oC) compared to Mx4 (1.56 ± 0.14oC) (p<0.01). There was no difference in the PCTR in M8 (1.77 ± 0.28oC) compared to M1 or Mx4 (p>0.05), and no difference between intact (1.78 ± 0.14oC) and restored (1.92 ± 0.08oC) teeth (p=0.38). There was no influence of dentin enamel thickness in the PCTR (p=0.19). Conclusion: PCTR was higher in light-cure bonding of brackets with primer, especially in M1. Light-cure bonding seems less invasive without primer.


INTRODUCTION
Clinical dental procedures can lead to pulp chamber temperature rise (PCTR). [1][2][3][4] Minor temperature elevations cause none or mild harm that can be reversed by means of physiological reactions of pulp tissues. Temperature increases above 5.5°C can represent a high risk of pulp inflammation and consequent pulp necrosis. 5,6 Light-cure bonding of brackets generates a wide range of heat variations, usually related to different light sources, exposure times, adhesive resin thickness, and exothermic reactions. 7-10 Even within the limits for irreversible damage in the pulp tissue, PCTR is undesired. 2,5 Standard bonding of brackets follows two light-curing steps: one for the primer, and another for the resin adhesive. The primer can enhance shear bond strength and offer better protection for etched enamel prisms, due to its thinner viscosity. 11 On the other hand, bracket bonding without the use of primer takes shorter time and decreases the exposure to moisture, which is a risk factor for bond failure. In vitro studies found equal shear bond strength in brackets bonded either using primer or not using primer. 12,13 Likewise, clinical studies reported no differences in bond failures between brackets bonded with or without the use of primer. 14-16 One-step bonding without primer saves time for light curing, and avoids cumulative heat that can lead to PCTR. 17 Moreover, bracket bonding without primer tends to reduce the amount of resin adhesive remaining after debonding. 16 Shorter time for resin adhesive removal prevents a new episode of PCTR. 4 Tooth conditions may play a role in the transfer of heat to the pulp chamber. Teeth have poor thermal conductivity; hence, the microstructure of the dentin-enamel junction functions to protect the pulp against temperature changes. 18 Thicker layers of dentin-enamel tissues appear to prevent PCTR. 19 Thus, intact teeth might be less vulnerable to PCTR than restored teeth.
Thermal conductivity of composite resins can induce a more aggressive reaction of pulp tissues. 18 Thus, the present study aimed to evaluate the PCTR during light-cure bonding of brackets, with and without the use of primer, in mandibular central incisors (M1), maxillary first premolars (Mx4), and mandibular third molars (M8), under both intact and restored conditions. The null hypothesis tested was that PCTR would not present significant differences in relation to bonding techniques, tooth types, or tooth conditions. M1, Mx4, and M8, with intact buccal surfaces and intact pulp chambers were included. After inspection, teeth with dentin lesions, large cavities, or surgical damage were excluded from the study. Ninety human teeth met the inclusion criteria and were stored in saline, at room temperature, up to four months, until the experiment.
Ten samples of each tooth type were randomly selected, prepared with dental cavities, subsequently restored with composite resin, then brackets were bonded using primer (n = 30).
The other 20 samples of each tooth type were divided in two groups: brackets bonded using primer (n = 30) and without primer (n = 30) ( Table 1)    The distal root of M8 and buccal root of Mx4 were sectioned at 5 mm from the cementum-enamel junction. The pulp chambers were cleaned using a dentin excavator, irrigated for 60 seconds with 2% sodium hypochlorite solution, rinsed with distilled water, and dried by oil-free air jet. The specimens were fixed in a prefabricated device, using self-curing acrylic resin, with the buccal surfaces exposed, to bracket bonding, and the root access unobstructed (Fig 1).
The thermocouple was placed against the buccal surface of the pulp chamber, stabilized with utility wax, and connected to a proper placement of the thermocouple (Fig 2). The specimens were then fixed in a glass plate using double-sided tape.  records (T1 − T0) (Fig 3). Specimen preparation and temperature assessments were performed in a random sequence, at room temperature.

ENAMEL-DENTIN THICKNESS
The teeth were cut in a mesial-distal direction using a diamond disk (1802.7016, KG Sorensen, Cotia, SP, Brazil) under refrigeration (Fig 3). The enamel-dentin thickness of the pulp chamber buccal wall was measured using a digital caliper (Starret, Athol, MA, USA), with 0.01-mm accuracy. This measurement was performed in order to control the possible effect of this covariate on the measured temperature.

STATISTICAL ANALYSIS
The Kolmogorov-Smirnov test and Levene test assessed the data distribution. Analysis of covariance (ANCOVA) was performed with a robust standard error, i.e., a covariance analysis of four factors (bonding technique, tooth type, tooth condition, and enamel-dentin thickness), with one factor being continuous (thickness). Data were analyzed in the SPSS software (version 18.0, Chicago, IL), at 5% significance level.

RESULTS
PCTR occurred in all specimens (overall mean = 1.94°C; range = 0.2-4.3°C) (Fig 4). PCTR showed statistically significant differences between bonding techniques (with primer vs. without primer; p = 0.02) and between tooth types (M1 vs. Mx4 vs. M8; p < 0.05) ( Table 2).   size, allowing the insertion into the root canal, as previously described. 4,9,22 PCTR measurement has a greater accuracy using thermocouples than using other methods, 23 due to the possibility of ensuring the proper position in the pulp chamber, using radiographs. 6,24 Light-cure bonding of brackets was carried out with the same LED source, in continuous mode, and maximum intensity.
LED in continuous mode produces less heat than in ramp or pulsatile modes. 24 PCTR continued for 20 seconds after the end of the light-curing process, due to possible cumulative heat in the pulp chamber. 17 Heat is the most severe stress supported by the dentin during dental procedures, inducing a concomitant response in the pulp tissues. 6,25 The main effects of heat on biological tissues are vasodilatation, exudation, and coagulative necrosis. Strong and rapid alternate expansion and contraction of intra-tubular fluid can damage the odontoblasts. 6 The present results showed that light-cure bonding of brackets the dentin's thermal insulation role. 1,8,18,20,27,28 However, in this study, dentin-enamel thickness had a low influence on PCTR.
The divergence of outcomes may be explained by the analysis, which was performed without separation according to different tooth types.
In the present study, restored teeth were bonded with primer and two steps of light-curing, which is a more invasive procedure. Despite that, PCTR was not significantly different (p=0.38) between intact teeth and restored teeth (Table 2). Thus, including an additional group with restored teeth and no primer proved dispensable. The thickness of adhesive layer between enamel and brackets was a confounding factor, which was controlled by a Gilmore needle. All dental restorations were performed with the same composite resin, in order to eliminate differences due to dental materials. Light-colored composite resins may show a greater temperature rise than darker ones during light-curing. Lighter shades favor light transmission, whereas darker shades are prone to light absorption. 28 This study raised a clinical implication: light-cure bonding of brackets with primer caused a PCTR higher than 3°C in 25% of M1 (maximum = 4.3°C). When the primer was not used, PCTR was lower than 3.4°C in 100% of M1 (Fig 4). In this sample, one-step light-cure bonding of brackets was less invasive, especially in M1. This is a relevant information due to other PCTR events during orthodontic treatment, such as brackets re-bonding, resin adhesive removal, and enamel polishing. 4,28 A limitation of this in vitro study is that heat conduction due to blood circulation inside the tooth and fluid movement inside the dentinal tubules was not considered. 20 In addition, the underlying periodontal tissues promotes heat dissipation in vivo, thus controlling the increase in pulp chamber's temperature. 27 Histopathological studies would enhance the current knowledge of thermal injury to the pulp during orthodontic bonding, to avoid unwanted outcomes such as pulpitis or pulp necrosis.